The present invention relates top the field of diagnostic imaging, and in particular the fields of nuclear imaging and Nuclear Magnetic Resonance (NMR) imaging.
In nuclear imaging, a radiopharmaceutical is introduced into the body of a patient. As the radiopharmaceutical decays, gamma rays are generated. The generated gamma rays are detected and used to construct a useful image. Positron Emission Tomography (PET) is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical is introduced into the body of a patient. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pairs of 511 KeV gamma rays emitted in opposite directions. Other examples of nuclear medicine devices include gamma cameras and Single Photon Emission Complete Tomography (SPECT) systems.
In NMR imaging, pulses of radio frequency energy are applied to the subject in the presence of an applied magnetic field. Under the influence of the radio frequency pulses, the magnetic moments of nuclei in the material of the subject are caused to precess about the direction of the applied magnetic filed to give detectable radio frequency signals. By mapping the difference between the radio frequency signals produced in different parts of a selected region of the subject, e.g. a slice through the subject, an image of the selected region of the subject may be obtained.
Nuclear imaging technology, such as gamma cameras and SPECT, is widely used in medicine. For example, in the detection of breast cancer, nuclear medicine (and particularly scintimammography), allows to distinguish cancerous tissue from healthy tissue. However, in many cases, the space resolution of the images obtained is not sufficient, and smaller features (typically less than 1.5 cm) cannot be detected. It would be thus very useful in the detection of very early stage tumors if one could increase the resolution of Nuclear medicine technology.
On the other hand, the space resolution achieved with MR imaging is much higher. But unfortunately, its ability to distinguish different tissues is very low. In particular, MR imaging does not allow to distinguish between benign and malign tumors.
In sum, typically, the MR imaging produces images in which anatomical structures are clearly visible, whereas nuclear imaging using devices such as gamma cameras, produces images in which the specificity of tissues can be characterized.
In view of the above, one can easily appreciate the advantages of combining the two methods, MR imaging and nuclear imaging. Ideally, the combined system would have the space resolution provided by MRI technology and also the specificity of nuclear imaging technology.
One could imagine two main approaches to combining these two very different technologies. In a first approach, MR images and Nuclear images are taken separately and the resulting images are combined in a post-processing step. In a second approach, the MRI device and the Nuclear device are combined into a single device, and both MR and Nuclear images are acquired simultaneously or quasi-simultaneously.
In line with the first approach, the particular combination of MR imaging and scintimammography (gamma camera) in a single image is implemented using the so-called “fusion image” technique. In fusion image, Nuclear images and MR images are acquired independently from one another. In a post-processing step, Nuclear images and MR images are overlaid to form “fusion” images or combined images. However, this technique does not eliminate the main shortcomings of each of these methods such as low spatial resolution and low signal-to-noise ratio for nuclear imaging and low specificity for MR imaging. The fact that fusion images retain or even enhance the problems of Nuclear and MR images is not surprising. Indeed, the images used to obtain a fusion image are acquired with completely different systems (Nuclear and MR) with no common coordinate systems, and thus images to be “fused” have different scales and projections. The “marker” method is generally used in conjunction with the fusion image technique. However, this “marker” method is not very precise and the images obtained are noisy.
The difficulty in combining Nuclear and MR images is compounded by the fact that the positions of the patient in nuclear imaging devices and MR devices are completely different. Further, Nuclear images and MR images are taken at different times and the time interval between images can be important (days to weeks).
One way to eliminate the above-mentioned problems, and in particular problems arising from the different coordinate systems and the difference in the position of the patient, would be to acquire Nuclear images and MR images simultaneously (or quasi-simultaneously in a biological time scale) with a combined Nuclear-MRI device, as suggested by the second approach mentioned earlier.
This was most successfully accomplished by combining a PET device and a CT scanner. This machine is the first medical imaging device to simultaneously and clearly reveal both, anatomical details, and metabolic processes within the body. The use of this combined machine in medicine is growing fast.
Because the MRI devices allow to see finer anatomical details (higher space resolution) than CT scanners, it would be advantageous to combine an MRI device with a Nuclear imaging device.
One of the main problems with attempting to combine nuclear imaging technology with MRI technology lies the in the sensitivity of the nuclear imaging device to strong magnetic fields. In particular, in the current state of the art, the photodetector module of a nuclear medicine device will not function properly if placed within a magnetic field. Indeed, virtually all existing nuclear imaging devices use photomultipliers (PMTs), and PMTs are very sensitive to magnetic fields. Because of this, direct combination of MRI-nuclear imaging cannot be accomplished in the same fashion as the CT-Nuclear combination.
In theory, solid-state photodetectors may be used in lieu of PMTs, since these photodetectors are immune to magnetic fields. However, solid-state photodetectors, including avalanche detectors are not fast enough and do not have the sensitivity required for Nuclear medicine applications. U.S. Pat. No. 4,939,464 discloses NMR-PET scanner apparatus wherein a PET detector is disposed within a magnetic imaging structure of an NMR device. To avoid interaction between the photodetector (specifically the photomultiplier tubes or PMT) of the PET detector, with the magnetic field generated by the magnetic imaging structure, the output of the PET detector is channeled to photodetectors that are shielded from the magnetic field.
U.S. Pat. No. 5,719,400 by Shao et al. discloses a high resolution detector array for gamma ray imaging, wherein the Mutlichannel PMTs (MC-PMTs) are capable of properly functioning in magnetic fields of up to 10 mT. This invention allowed Shoa et al. to combine an MRI device with a single-slice PET scanner, the PET scanner using LSO crystals optically coupled to three MC-PMTs each disposed about 4 m away from the center of the magnetic field. In this region, the magnetic field falls below a certain magnitude which is adequate for proper functioning of the MC-PMTs. The work of Shoa et al. demonstrated that it may be possible to achieve simultaneous acquisition of PET and MRI images without noticeable quality loss in either the PET or MR images.
However, the Shoa et al. system has limitations which do not allow to exploit the full potential of a combined nuclear imaging-MRI system. Some of these limitations are listed below:                Because of the length of the optic fibers used (up to 4 m), large losses of light occur in the fibers.        A compact device cannot be made since the PMTs have to be placed away (up to 4 m) from the rest of the device.        The 8 elements of the detector are located directly in the RF coil of the MRI device and magnetic interference cannot be prevented.        Insufficient main magnet field screening.        Ineffective image reconstruction technique (back projection).        MC-PMTs are very expensive.        
What is needed is a combined MR-nuclear imaging device which is relatively compact and that can quasi-simultaneously acquire both MR and Nuclear images. Further, it is needed a Nuclear device which is immune to magnetic fields so that a functional combined MR-nuclear imaging device may be built.